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Abstract: This work presents the methodological study, processing and optimization of novel, technologically acceptable procedure for in situ coating of polypropylene (PP) mesh (used for hernia treatment) with physico-chemically, mechanically and micro-structurally different gelatin (GEL) scaffolds to assess implant composite biocompatibility impact. In order to systematically follow the experimental work progress and respective achievements, whole research path is subdivided into three main sections. In the first section, the procedure for fabrication of gradiently micro-porous GELscaffolds on the cryo-unit’s cooling plate surface, using spatiotemporal and temperature- controlled gelation and freezing, followed by lyophylizaton was studied. Subsequently, cross-linking procedure using different molarities of reagents (EDC and NHS) and reaction media (100% PBS or 20/80% PBS/EtOH mixture) was performed for variable time extensions (1-24 h), rendering scaffolds physico-chemical properties. In this way, scaffolds with micro-structures having porosity gradient from 100 µm to 1000 µm and pores with rounded to ellipsoid morphology were formed, which, in combination with ethanol (EtOH) addition in cross-linking media modulates the swelling capacity towards twice lower percentages (~600%) comparing with scaffolds cross-linked in 100% PBS. Whilst the presence of EtOH reduce the cross-linking kinetic by retaining the scaffolds’ micro-structure formed during freezing, the 100% PBS and higher EDC molarity resulted in 40% cross-linking degree, being expressed as a thermal resistance up to 73 °C. The presented integral fabrication procedure was shown to allow tuning of both, the physical and micro-structural properties of scaffold, utilized in preparation of materials for specific biomedical applications. In the second part, the complex relation between surface and interface-related physico-chemical properties and gradient micro-structuring of 3D GELscaffolds, being fabricated by simultaneous temperature- controlled freeze-thawing cycles and in situ cross-linking using variable conditions (pH and molarity of carbodiimide reagent) and fibroblast cells viability (by tracking of their spreading and morphology) was established. Rarely- populated cells with rounded morphology and small elongations were observed on scaffolds with apparently negatively- charged surface with a lower cross-linking degree (CD) and consequently higher molecular mobility and availability of cell-recognition sequences, in comparison with the prominently- elongated and densely- populated cells on a scaffold’s with positively- charged surface, higher CD and lower mobility. Surface micro-structure effect was demonstrated by cell’s vacuolization and their pure inter-communication being present on scaffold’s bottom side with smaller pores (25±19 µm) and thinner pore walls (9±5 µm), over the air- exposed side with twice bigger pores (56±38 µm) and slightly thicker pore walls (12±6 µm). Strong correlation of preparation conditions (pH and reagents molarity) with CD (r2=0.96) and moderate correlation with local molecular mobility (r2 =-0.44), as well as micro-structure features being related to temperature gradient, imply on possibility to modulate scaffold’s properties in a direction to guide cell’s viability and most likely its genotype development. The third part presents an innovative strategy for the fabrication of bio-active PPmesh-GELscaffold composites with a potential for abdominal hernia treatment, where mesothelial cells in-growth have to be stimulated together with fibroblasts on-site proliferation, while formation of fibrin-developing, viscera-to-abdominal wall adhesions should be reduced, together with bacteria- related infections. In this respect, the plasma pre-activated PPmesh was coated with micro-structured GELscaffold, with pore size in 50 µm to 100 µm range at the upper-side and loosely- porous network at the composite bottom side, being modulated by sample thickness and freezing end- temperature applied. Simultaneously, the
Keywords: gelatin, targeted cross-linking, controlled freezing, gradiental micro-porosity, scaffold, surface and interface chemistry, physico-mechanical properties, polypropylene mesh, composite, biocompatibility.
Published in DKUM: 07.05.2014; Views: 2355; Downloads: 196
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Abstract: This paper presents a case study, examining the influence of a sharp bimaterial interface on the effective crack driving force in a fracture mechanics specimen. The inhomogeneity of the elastic modulus in linear elasticand non-hardening and hardening elastic-plastic bimaterials is considered. The interface is perpendicular to the crack plane. The material properties and the distance between the crack tip and the interface are systematically varied. The effect of the material inhomogeneity is captured in form of a quantity called "material inhomogeneity term",▫$C_inh$▫. This term can be evaluated either by a simple post-processing procedure, following a conventional finite element stress analysis, or by computing the J-integral along a contour around the interface, ▫$J_int$▫. The effective crack driving force,▫$J_tip$▫, can be determined as the sum of ▫$C_inh$▫ and the nominally applied far-field crack driving force, ▫$J_far$▫. The results show that ▫$C_inh$▫ can be accurately determined by both methods even in cases where ▫$J_tip$▫-values are inaccurate. When a crack approaches a stiff/compliant interface,▫$C_inh$▫ is positive and ▫$J_tip$▫ becomes larger than ▫$J-far$▫. A compliant/stiff transition leads to a negative ▫$C_inh$▫, and ▫J_tip$▫ becomes smaller than ▫$J_far$▫. The material inhomogeneity term, ▫$C_inh$▫, can have the same order of magnitude as ▫$J_far$▫. Based on the numerical results, the dependencies of ▫$C_inh$▫ on the material parameters and the geometry are derived. Simple expressions are obtained to estimate ▫$C_inh$▫.
Keywords: mechanics of structures, fracture toughness, inhomogeneous materials, J-integral, crack driving force, interface, material force
Published in DKUM: 01.06.2012; Views: 1720; Downloads: 37
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Abstract: A material inhomogeneity in the direction of crack extension causes a difference between the near-tip crack driving force, Jtip, and the nominally applied far-field crack driving force, Jfar. This difference can be quantified by a material inhomogeneity term, Cinh, which is evaluated by a post-processing procedure to a conventional finite element stress analysis. The magnitude of the material inhomogeneity term is evaluated for cracks in an inhomogeneous welded joint made of a high-strength low-alloy steel. Both a crack proceeding from the under-matched (UM) to the over-matched (OM) and from the OM to the UM weld metal are treated. The effects of the inhomogeneity of the different material parameters (modulus of elasticity, yield strength, and strain hardening exponent) on Cinh and Jtip are systematically studied. The results demonstrate that the material inhomogeneity term is primarily influenced by the inhomogeneity of the yield strength. A crack growing towards an OM/UM interface experiences an accelerated crack growth rate or a pop-in, an UM/OM interface leads to a reduced crack growth rate or a crack arrest. The application of global assessment methods of the mismatch effect which are included in the Engineering Treatment Model (ETM) or in the Structural Integrity Assessment Procedures for European Industry (SINTAP) is discussed.
Keywords: crack driving force, material inhomogeneity, mismatched weld, interface, J-integral, finite element modeling
Published in DKUM: 31.05.2012; Views: 1754; Downloads: 91
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Keywords: GPRS, PLK, SCADA, VPN, komunikacijski vmesniki, modbus TCP/IP, stroški, GPRS, PLC, SCADA, VPN, communication interface, modbus TCP/IP, costs
Published in DKUM: 26.07.2007; Views: 3924; Downloads: 0